test: wait for the setup compile deterministically in workspace init

[magnus-madsen]

Problem

CI still shows intermittent VS Code extension test failures (example run):

1) RenameProvider Should rename variable:      Error: No result.
2) RenameProvider Should rename variable-use:   Error: No result.

Error: No result. means the vscode.executeDocumentRenameProvider request came back empty — the server's index had no symbol at that position because the workspace hadn't been compiled yet when the test ran.

Root cause

#545 converted in-test file mutations to wait on lsp/check via awaitCheck, but the per-suite workspace swap in init still used fixed sleeps (processFileChange: sleep(1000) → awaitIdle → sleep(1000)).

awaitIdle (flix.allJobsFinished) resolves immediately whenever the compiler's queue is momentarily empty — which it is if the file-system watcher hasn't fired yet after copyDirContents. Under CI load, init could return before the check triggered by copying the suite's files had finished compiling them.

RenameProvider (suite #12) does no in-test mutation, so it never benefits from awaitCheck — its correctness depends entirely on init leaving the workspace compiled and idle. Hence the residual flakiness.

Fix

Make the setup path deterministic, the same way the mutation path already is:

• settleAfterChange(baseline) waits for checkCount to advance past a baseline captured before the change (proving the watcher fired and a check completed), then repeatedly drains to idle until the check count is stable across a reconcile-debounce window (the client schedules a 300 ms-debounced reconciliation that can enqueue a follow-up check).
• init baselines and settles separately for the clear and the copy, and waits for the removals to be observed before copying — so VS Code reports the new files as creates rather than coalescing a delete-then-create of the same path into a single (unhandled) change event.
• The first suite (extension not yet active) has no watcher/baseline; its copied files are picked up by the initial workspace scan on ext.activate(), and settleAfterChange(0) waits for that compile.
• Removes the now-unused processFileChange.

Net effect: the fixed ~4 s of per-suite sleeps are replaced by a deterministic wait anchored to observable compiler progress (typically faster, and no longer racy).

Testing

npm run build and tsc --build ./test pass. The change is timing-only in the test harness; validated end-to-end on CI (local single runs can't reproduce load-dependent flakiness).